A known process for viscbreaking polyolefins such as polypropylene is extrusion at about 180-260.degree. C. in the presence of an organic peroxygen compound ("peroxygen"). A typical organic peroxygen used commercially for this purpose is 2,5-dimethyl-2,5-di(t-butylperoxy) hexane, sold by Aldrich Chemical Co. as "Lupersol 101". This peroxygen is a liquid boiling at 115.degree. C. and having a minimum active oxygen content of 9.92%. Dispersion of this organic peroxygen in the polypropylene or other polyolefin is usually assisted by dilution of the peroxygen in mineral oil, coating of the polyolefin particles, and/or metered injection directly into the extruder. Although Lupersol 101 peroxygen is effective in lowering the viscosity of polyolefins and producing a relatively narrow molecular weight distribution, it leaves an odor in polyolefin from decomposition products such as tetrabutyl alcohol, thus requiring vacuum venting and stabilizers at predetermined levels to reduce the degradation of the polyolefin by oxgen. Lupersol 101 also decreases the polydispersity index of polyolefin, e.g., in the case of polypropylene to about 3.5-6.0. Polyolefin yellowing may also be induced by the peroxygen, requiring a lower extrusion temperature, a nitrogen blanket and/or stabilizers to eliminate or minimize the coloration. The venting, stabilization and color control procedures, separately or when combined, add significantly to the complexity and cost of the viscbreaking process.
Organic peroxides are known to leave toxic decomposition products in polyolefins such as polypropylene during the viscbreaking process. Moreover, due to their organic nature, such peroxides require special handling precautions. Accordingly, safe handling is a significant concern with organic peroxides.
A free radical mechanism is believed to account for polyolefin degradation by application of peroxygens. Initially, the peroxygen decomposes to produce free radicals. These free radicals then abstract hydrogen from the tertiary carbon of the polyolefin backbone to form radicals on the polymer. This results in chain cleavage of the formed free radicals. The process can be terminated by recombination of the polymer free radicals. Oxygen, processing stabilizers and antioxidants are known to interfere with the polymer degradation process due to competing radical reactions. It is thus important to control the parameters of the reaction, including purity of reactants, the environment of the degradation process, and process conditions.